This invention relates generally to the extractive metallurgy of ores, and, more associated values, especially cobalt and magnesium, from ores.
Metal values are often found in nature as compounds, present in small fractions in ores, mixed with a wide variety of impurities. To recover the metal values of interest for subsequent use, they must be removed from the ores and concentrated, refined and processed. The removal of the metal values from ores, termed extractive metallurgy, is desirable accomplished so that the metal values of interest are removed with great selectivity, as removal of impurities can greatly complicate the subsequent concentration and refinement procedures. Thus, extraction processes should be highly selective, so that only the metal values of interest are extracted from the ore, leaving behind the impurities in the residue to be discarded.
Nickel, a commercially significant metal, is found together with cobalt and other metals in a variety of types of nickel-bearing ores that are mined throughout the world. In one well established process, nickel and cobalt are extracted from an ore by mixing the ore with sulfuric acid to leach the nickel values into the sulfuric acid solution and form a leachate. Some cobalt and other metallic values such as magnesium are typically also dissolved into the leachate. The metallic values are then separated from the leachate by any of several processes, such as neutralization of the leachate with a base and precipitation of the nickel and cobalt values as hydroxides. The precipitated values are then gathered and refined into the respective metals.
The extraction process just described is typically carried out at elevated temperatures and pressures. The ore and sulfuric acid are introduced into an autoclave, which is heated to about 200.degree. C. to about 300.degree. C. and pressurized to about 200 to about 2000 pounds per square inch. These conditions promote the nickel extraction, as compared with leaching at ambient temperature and pressure. Because the nickel and cobalt are present in the ore in low concentrations such as a few percent by weight at most, large amounts of ore must be processed through autoclaves in this extraction process to obtain commercial quantities of nickel.
Ideally, the leaching process would attach and dissolve only the desired metal values into the leachate, but in practice the sulfuric acid also dissolves quantities of the impurities into the leachate. These impurities are then present in the leachate during subsequent steps, and either must be removed or tolerated during concentration and refining of the metal values.
Aluminum and iron are impurities found in many nickel-bearing ores, often in concentrations as great or greater than the desired nickel values. When the leachate is neutralized to precipitate the nickel values, the aluminum and iron impurities form precipitated hydroxides and basic sulfates that are gelatinous in nature, messy, and difficult to work with. Although high-temperature leaching tends to reduce the amount of the impurities coextracted with the nickel values, the precipitated impurities interfere with the removal of the precipitated nickel and cobalt values. It is not uncommon that the amount of undesired precipitated impurities is about as great as the amount of the desired nickel-containing precipitate, and the presence of the impurities can be a major obstacle to the complete recovery of the desirable metal values. As an example, in one Indonesian lateritic ore the nickel content is about 1.6 percent, and the aluminum content about 1.4 percent.
Selection of the proper leaching conditions can reduce the amount of extracted aluminum impurities. A number of other techniques have been developed for minimizing the amount of the extracted impurities, and some of these techniques are applied during the extraction operation. Unfortunately, there are side effects of some of the techniques that are worse than the problem partially solved by their use. Other techniques are effective in avoiding iron or aluminum in the leachate, or controlling their presence and harmful effects, but are expensive to use in large-scale operations or have limited utility when both aluminum and iron are present. For example ferric iron can be removed from aqueous solution by precipitation at atmospheric pressure and near-boiling temperature as crystalline jarosite, but the presence of aluminum complicates the process as it cannot be removed as crystalline alunite at atmospheric pressure. Aluminum can be removed as alunite in a second leaching step at lower temperatures than used in the nickel leaching, but requires the use of expensive equipment to withstand the pressure and corrosive conditions of the process.
Experience has shown that the aluminum content of the leachate must be reduced below about 0.5 grams per liter to avoid the problems associated with its presence. If the aluminum is present in greater amounts, there must be a separate, costly treatment of the leachate to reduce the aluminum to levels acceptable in subsequent operations. On the other hand, if the aluminum content is below about 0.5 grams per liter, its presence is tolerable in subsequent operations. Thus, the aluminum impurity level of about 0.5 grams per liter is critical, in that the presence of larger amounts of aluminum require costly additional processing steps to reduce the aluminum content to less than that level.
There therefore exists a need for a process for extracting nickel values from nickel-bearing ores having aluminum impurity, wherein the presence of aluminum is reduced to a level of less than about 0.5 grams per liter in the leachate. Such a process must be inexpensive to use and desirably would be fully compatible with the presently used high-pressure leaching of the nickel-bearing ore with sulfuric acid. The present invention fulfills this need, and further provides related advantages.